Integrated circuits with more and more functionality are needed to make advanced devices. Often multiple types of transistors such as logic MOS transistors, memory MOS transistors, input/output MOS transistors, high voltage MOS transistors and analog MOS transistors are needed on the same integrated circuit.
Analog MOS transistors may operate at higher voltages than logic MOS transistors. To form a reliable higher voltage MOS transistor in an integrated circuit typically requires sufficient overlap between the high voltage gate and high voltage drain diffusion to reduce the peak electric field near the gate corner in order to provide sufficient channel hot carrier CHC) reliability. Sufficient overlap is commonly provided by source/drain extensions which extend under the high voltage gate. Halo implants, which are angled implants of an opposite doping polarity from the source/drain extensions, may be used on logic MOS transistors to combat short channel effects. Halo implants may produce a significant slope in the drain voltage versus drain voltage curve of a MOS transistor, which may not be a problem for logic MOS transistors but may present a serious problem for analog MOS transistors. A slope as close to zero as possible may desirable for analog MOS transistors. Thus, analog MOS transistors may be added to the integrated circuit by adding separate photolithography and implantation steps to form source and drain extensions without halo implants. The additional photolithography and implantation steps undesirably increase the fabrication cost of the integrated circuit.